DUAL DAY LENGTH REQUIREMENTS 317 



LD induction in Cestrum noctiirmtm, a LSDP, differs from that in 

 LDP: continuous light is not as effective as a 16-hr photoperiod in 

 Cestrum (Sachs, 1956b), whereas in LDP no combination of light and 

 dark has been found to be more favorable than continuous light 

 (Lang, 1952) ; the temperature of the photoperiod has a greater effect 

 than the associated dark period in Cestrum (Sachs, 1956c), whereas 

 in the two LDP for which the effect of temperature has been investi- 

 gated the reverse was found to be true (Lang and Melchers, 1943; 

 Liverman and Lang. 1952). These differences have been discussed 

 elsewhere (Sachs. 1956b,c), and attempts were made to compare the 

 various experiments; owing to great differences in experimental tech- 

 nique the discussion was highly speculative. For this reason it will not 

 be repeated here, and the interested reader is referred to the original 

 papers. 



Of what help are the dual day length-requiring plants in understand- 

 ing the photoperiodic control of floral initiation? Before considering 

 this problem two assumptions will be made in order to facilitate the 

 discussion. There have been an increasing number of reports of inter- 

 specific and intergeneric flower-inducing grafts between plants of the 

 same and different photoperiodic type (Lang, 1952; Okuda, 1953; 

 Khudairi and Lang, 1954), and the first assumption is that the floral 

 stimulus is the same in all plants. The second is that the synthesis of 

 the stimulus proceeds along the same pathway in all plants; although 

 there is no evidence supporting this assumption, it is one that has 

 proved valid for the biosynthesis of a number of compounds found in 

 plants. It is recognized that both assumptions may be oversimplifica- 

 tions of the actual situation. 



In LSDP and SLDP both LD and SD induction occur in the same 

 plant, and, therefore, there must be at least two photosensitized reac- 

 tions involved in the synthesis of the floral stimulus. In Cestrum it has 

 been shown that the product of LD induction is not translocated from 

 the treated leaves, whereas shortly after SD induction the floral stimu- 

 lus moves to the axillary and terminal buds; hence, in a physiological 

 sense, the products of LD and SD induction are easily separable. On 

 this basis alone it is reasonable to ask whether LD induction in the 

 LDP affects the same or a different step in the synthesis of the floral 

 stimulus from that affected by SD induction in SDP. Furthermore, a 



